Used as conventional disc device for recording digital data are, for example, a magnetic disc, which read data by detecting a magnetic change, and an optical disc, which read data by detecting light intensity of reflected light that is caused by projecting a beam, such as a laser beam.
There are an exchangeable (medium-exchangeable) magnetic disc, such as a floppy disc, a ZIP disc, and JAZ (Registered Trademark) disc and a non-exchangeable (non-medium-exchangeable) magnetic medium like a hard disc drive. In order to increase a recording density of the magnetic disc, it is necessary to reduce a space between a recording surface of the magnetic disc and a magnetic head. However, the reduction of the space makes the magnetic disc more susceptible to an adverse effect of dust. Especially, for the exchangeable magnetic disc, even if the exchangeable magnetic disc is contained in a cartridge, there is a high tendency of invasion of the dust into an inside of the cartridge, thereby posing a problem in terms of reliability of the exchangeable magnetic disc.
On the other hand, in case of the hard disc drive, which has an arrangement in which a hard disc is contained in a sealed box container, thereby not allowing the hard disc drive to be medium exchangeable, an inside of the apparatus is protected against the invasion of the dust. This has intensively encouraged the hard disc drive to have a higher recording density. However, since the hard disc drive has the arrangement not for the medium-exchange, it is necessary to install an extra hard disc drive or to transfer stored data to an exchangeable recording medium, when a storage capacity of the hard disc drive is full.
On the contrary, the optical disc has such an arrangement in which a distance between (a) a surface of the optical disc for recording information and (b) an optical head for reading the information is more than or equal to 1 mm. In other words, the optical disc is not significantly affected by the dust. Because of this, the optical disc has a comparably high recording density while the optical disc is exchangeable, thereby attaining a high popularity.
Recently, various products are provided with a disc apparatus for the optical disc. For example, if the optical disc is mounted in a portable product such as a notebook personal computer, or a portable optical disc player, it is more preferable that the disc apparatus is thinner.
A conventional optical disc has an arrangement in which a laser beam for recording/reproducing is focused on a recording surface through a substrate (made of polycarbonate, for example). For this reason, an objective lens for focusing the laser beam for reproducing should has a comparatively large diameter, so that a thickness of the substrate can be corresponded with a focal point distance. Because of this, it is necessary that a laser beam directed to the objective lens should have a comparatively large diameter, while a mirror, which reflects a light beam emitted from a laser beam source to the objective lens, should have a large size. In short, the optical head need a certain thickness. Thus, it is difficult to make the optical disc drive thinner.
Moreover, because the optical head has a comparatively heavy weight for the same reasons, the optical disc drive has a difficulty to attain a high-speed random access, unlike the hard disc drive. Thus, it is also a problem for the optical disc drive that the optical disc drive requires a longer time for reading and writing data, compared with the hard disc drive.
Furthermore, the conventional optical disc for reading and writing has an arrangement that a disc substrate having a recording film is contained in a cartridge so as not to be dusty and dirty. In order to realize a thinner disc drive, the optical disc having the arrangement is also demanded to be thinner.